7.1 - Human Endocrine System

Endocrine System and Its Functions

The endocrine system coordinates body functions involving chemical substances.
An endocrine gland is a ductless gland that secretes hormones directly into the bloodstream.
A hormone is an organic chemical substance secreted by an endocrine gland. It is a type of protein, secreted in small quantities and carried through blood circulation.
Hormones stimulate specific tissues or organs and produce a slow response with a prolonged effect.
A target organ has a suitable receptor that recognises and responds to a specific hormone.

Hormone coordination pathway:

Emergency response when danger is detected:

Hormones increase heartbeat, rate of breathing, blood pressure, blood glucose level and rate of metabolism.
The heart pumps more blood, supplying more oxygen and glucose to muscles.
More stored glycogen is converted into glucose.
Muscles obtain more energy, enabling the person to run away from danger.

Endocrine gland and position	Hormone and function
Pituitary gland — below the cerebrum; the master gland because its hormones control other endocrine glands	Antidiuretic hormone (ADH): controls the quantity of water reabsorbed by the kidneys. Growth hormone (GH): stimulates growth in children; maintains healthy body composition, muscle mass and bone mass in adults.
Thyroid gland — in front of the trachea	Thyroxine: controls the rate of metabolism and the physical and mental development of children.
Adrenal glands — above the kidneys	Adrenaline: prepares the body for an emergency by increasing the rate of metabolism, heartbeat and blood glucose level, and by dilating the pupils.
Pancreas — behind the stomach	Insulin: controls blood glucose level by converting excess glucose into glycogen for storage in the liver.
Ovaries — a pair in the female pelvic space	Oestrogen: controls female secondary sexual characteristics such as breast development and broadening of the hips, stimulates ovum production, and prepares the uterus for embryo implantation. Progesterone: maintains the thickness of the uterine wall for embryo implantation.
Testes — a pair protected by the scrotum	Testosterone: controls male secondary sexual characteristics such as a deep voice and moustache growth, and stimulates sperm production.

Hormone secretion must be balanced with its function.
Over-secretion or under-secretion causes hormonal imbalance and may lead to disease.
Causes include an unhealthy lifestyle, an unhealthy diet and exposure to dangerous rays.

Deficiency: low rate of metabolism, inability to tolerate cold, stunted physical and mental development in children (cretinism), less energy in adults (myxoedema), tendency to become fat and goitre.
Excess: high rate of metabolism, sweating and constantly feeling hot, difficulty sleeping, very good appetite, tendency to become thin, enlarged thyroid gland, protruding eyeballs and swollen neck.

Testosterone deficiency: delayed puberty and low sperm count.
Excess testosterone: male characteristics develop in women.
Oestrogen deficiency: development of female secondary sexual characteristics is disrupted.
Excess oestrogen: female characteristics develop in men.
Progesterone deficiency: menstrual problems, headache, constipation and miscarriage.

Deficiency: poor water reabsorption at the collecting duct in the kidney, excessive urine production, excessive thirst and diabetes insipidus.
Excess: headache and dizziness.

Deficiency: glucose cannot be converted into glycogen, blood glucose level increases and diabetes mellitus occurs.
Excess: low blood glucose level, hypoglycaemia and excessive thirst.